为什么得不到复制后的结果，到底错在哪了？
#include<stdio.h>
void main()
{
    void copy_string(char * from,char * to);
    char * a="I am a teacher.";                            //    char from[]="I am a teacher.",* a=from;
    char * b="you are a student.";                        //    char to[]="you are a student.",* b=to;
    printf("string a=%s\nstring b=%s\n",a,b);
    printf("copy string a to string b:\n");
    copy_string(a,b);
    printf("\nstring a=%s\nstring b=%s\n",a,b);
}

void copy_string(char * from,char * to)
{
    for(;* from!='\0';from++,to++)
        * to=* from;
    * to='\0';
}

首先输出数组a、b，然后把数组a复制到数组b，再输出数组a、b；但为什么不可以用下面的代码

char * b="you are a student.";

b的类型不是指针变量么

b指向字符串"you are a student."的首地址